# [SI-LIST] Re: characteristic impedance at DC

• To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
• Date: Tue, 10 Apr 2012 00:23:05 +0000

```Sorry, my bitmap drawing didn't make it through
the reflector.  The drawing had two boxes.  The
box on the left side contained a Thevenin driver
and the box on the right side contained a question
mark.

====================================================

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Muranyi, Arpad
Sent: Monday, April 09, 2012 7:03 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: characteristic impedance at DC

I have a problem with the statements that the impedance
is very low, on the order of a few milliOhms or so at

low frequencies or DC.

Consider this circuit:

[cid:[email protected]
/* <![CDATA[ */
(function(){try{var s,a,i,j,r,c,l,b=document.getElementsByTagName("script");l=b[b.length-1].previousSibling;a=l.getAttribute('data-cfemail');if(a){s='';r=parseInt(a.substr(0,2),16);for(j=2;a.length-j;j+=2){c=parseInt(a.substr(j,2),16)^r;s+=String.fromCharCode(c);}s=document.createTextNode(s);l.parentNode.replaceChild(s,l);}}catch(e){}})();
/* ]]> */
]

How would you measure the impedance in the box on the right?

You would measure it by measuring the voltage and current

that is in the wire connecting the two boxes, right?  What

would you get if the box contained a capacitor and the source

on the left would be a DC source?  You would measure the full

voltage of that source with 0 current, i.e. the impedance

would be infinite.  Why would the T-line exhibit a few milli

Ohms, if the insulating material is not leaking significantly?

I think that idea comes from measuring the series resistance

of the T-line, from its left end to its right end.  But that

is not the load impedance a driver (such as the box on the left

in the above diagram) would see...

Thanks,

===============================================================

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Todd Westerhoff
Sent: Monday, April 09, 2012 5:54 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: characteristic impedance at DC

Characteristic Impedance (sometimes called surge impedance) is the

impedance seen by a propagating wavefront.  It's been covered at length

multiple times, including this overview in Wikipedia:

http://en.wikipedia.org/wiki/Characteristic_impedance

DC impedance from a typical transmission line ranges from nil to a few

ohms, depending on the line geometry and length.  It's amenable to direct

physical measurement in many cases.  Here's a device that can be used to

affordably make such measurements with 4 digits of precision:

Hope that helps,

Todd.

Todd Westerhoff

VP, Software Products

Signal Integrity Software Inc. * www.sisoft.com<http://www.sisoft.com>

6 Clock Tower Place * Suite 250 * Maynard, MA 01754

(978) 461-0449 x24  *  twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>

"I want to live like that "

-Sidewalk Prophets

-----Original Message-----

From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx]<mailto:[mailto:si-list-bounce@xxxxxxxxxxxxx]>

On Behalf Of mohammad haaeri

Sent: Monday, April 09, 2012 6:31 PM

To: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>

Subject: [SI-LIST] characteristic impedance at DC

Hi,

What is the characteristic impedance of a transmission line at DC? If you

are saying Z0=sqrt(Rdc/Gdc) at DC, since Gdc=0, and Rdc is not zero,

therefore Z0 is infinite. Is it correct?

How does behavior of L, R, G, and C (line parameters) change vs. frequency

(at low and DC, and at very high frequency)?

Can Z0=sqrt(R+jwl/G+jwc) be used for all frequencies?

Thanks,

------------------------------------------------------------------

To unsubscribe from si-list:

si-list-request@xxxxxxxxxxxxx<mailto:si-list-request@xxxxxxxxxxxxx> with
'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:

http://www.freelists.org/webpage/si-list

For help:

si-list-request@xxxxxxxxxxxxx<mailto:si-list-request@xxxxxxxxxxxxx> with 'help'
in the Subject field

List forum  is accessible at:

http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:

http://www.freelists.org/archives/si-list

Old (prior to June 6, 2001) list archives are viewable at:

http://www.qsl.net/wb6tpu

------------------------------------------------------------------

To unsubscribe from si-list:

si-list-request@xxxxxxxxxxxxx<mailto:si-list-request@xxxxxxxxxxxxx> with
'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:

http://www.freelists.org/webpage/si-list

For help:

si-list-request@xxxxxxxxxxxxx<mailto:si-list-request@xxxxxxxxxxxxx> with 'help'
in the Subject field

List forum  is accessible at:

http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:

http://www.freelists.org/archives/si-list

Old (prior to June 6, 2001) list archives are viewable at:

http://www.qsl.net/wb6tpu

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List forum  is accessible at:
http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:
http://www.freelists.org/archives/si-list

Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List forum  is accessible at:
http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:
http://www.freelists.org/archives/si-list

Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu

```